CONFIG_NOMMU_INITIAL_TRIM_EXCESS: Turn on mmap() excess space trimming before booting

General informations

The Linux kernel configuration item CONFIG_NOMMU_INITIAL_TRIM_EXCESS:

• prompt: Turn on mmap() excess space trimming before booting
• type: int
• depends on: ! CONFIG_MMU
• defined in mm/Kconfig
• found in Linux kernels: 2.6.30–2.6.39, 3.0–3.19, 4.0–4.20, 5.0–5.19, 6.0–6.12

Help text

The NOMMU mmap() frequently needs to allocate large contiguous chunks of memory on which to store mappings, but it can only ask the system allocator for chunks in 2^N*PAGE_SIZE amounts - which is frequently more than it requires. To deal with this, mmap() is able to trim off the excess and return it to the allocator.

If trimming is enabled, the excess is trimmed off and returned to the system allocator, which can cause extra fragmentation, particularly if there are a lot of transient processes.

If trimming is disabled, the excess is kept, but not used, which for long-term mappings means that the space is wasted.

Trimming can be dynamically controlled through a sysctl option (/proc/sys/vm/nr_trim_pages) which specifies the minimum number of excess pages there must be before trimming should occur, or zero if no trimming is to occur.

This option specifies the initial value of this option. The default of 1 says that all excess pages should be trimmed.

See Documentation/admin-guide/mm/nommu-mmap.rst for more information.

Hardware

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The data is retrived from:

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• Linux Kernel Driver DataBase (LKDDb)

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